There is increasing interest by government agencies and the general public in protecting individuals against the harmful effects of toxic materials. Respirators of the type employing filter cartridges or canisters are commonly used for protection against respiratory hazards which include toxic vapors and gases. The respirator or only the cartridge is replaced when the end of service life indicator or device incorporated therein indicates insufficient adsorbent capacity remaining in the cartridge to justify its further or additional use.
Monitoring of personal exposure to hazardous materials is the subject of a number of studies of which the following are examples: Natusch, Sewell and Tanner, "Determination of H.sub.2 S in Air--An Assessment of Impregnated Paper Tape Methods", Analytical Chemistry, volume 46, page 3 (1974); Schnakenberg, "A Passive Personal Sampler for Nitrogen Dioxide", Bureau of Mines Technical Progress Report 95 (1976); Ray, Carroll and Armstrong, "Evaluation of Small Color-Changing Carbon Monoxide Dosimeters", Bureau of Mines Rep. Invest. (1975); Palmer; "Personal Samplers for CO, NO and NO.sub.2 in Air", Bureau of Mines Report OFR 92-77 (1977) and Nichols, "Reactive Tapes for Automatic Environmental Analysis, Personal Vapor Monitoring Badges for Industrial Workers", National Science Foundation Report NSF/RA-780039 (1978).
Colorimetric end of service life indicators are known in the art. U.S. Pat. No. 4,154,586 (and related German and British Pat. Nos. 2,758,603 and 1,554,542, respectively) provide a visual means for indicating when vapor/gas cartridges have exhausted their capacity to provide respiratory protection at or below a hazardous concentration level. The indicator material comprises a catalytic agent for enhancing activation and reaction of the indicator agent.
U.S. Pat. No. 1,537,519 discloses a ribbed window-type or a transparent canister wall respirator wherein the viewable absorbent is impregnated with an indicator. The patentee also discloses use of an indicator test strip (such as litmus paper) but only with the window-type canister. The patentee states that when use of his respirator is interrupted, the indicator may resume the color or appearance of the unspent indicator.
Another window-type canister or cartridge with color changing indicator means incorporated therein is disclosed in U.S. Pat. No. 3,966,440.
U.S. Pat. No. 4,155,358 discloses a valveless chemical cartridge respirator for vinyl chloride monomer comprising a colorimetric end of service life indicator disposed across the path of air intake at the entrance of the cartridge.
U.S. Pat. No. 4,146,887 discloses a gas or vapor sensing alarm device in an air purifying respirator for warning the wearer of hazardous levels of gases or vapors penetrating through the respirator cartridge.
These prior art end of service life indicators generally utilized granular colorimetric indicator particles or other probes located in the sorbent bed. In contrast to the prior art where indicator reliability may be reduced due to its incorporation in a localized pocket or in a window in the sorbent bed, the present invention utilizes an indicator means which reveals the remaining capacity of the entire sorbent bed rather than the condition of a small volume near the probe or window.
In addition, the colorimetric indicators useful in the present invention undergo irreversible color changes when subjected to gases to be detected.